Note: Allpar does not take responsibility for the veracity of any information or opinions here, does not claim expertise, may not have verified or performed the fixes, repairs, or modifications, and is not responsible for any consequences. Please proceed at your own risk.
Overheating can be extremely bad for a car. Back in the days of carburetors, when engines were made of steel, overheating was much more common, and less damaging. But the modern engine has far less tolerance, with its aluminum blocks and, often, aluminum heads.
Many head gaskets have been blown out by overheat issues. Many heads have been cracked. These are not cheap repairs. Therefore, be vigilant and watch that heat gauge (thermometer). If your car starts to overheat, pull to the side of the road, and let it idle (do not give it a fast idle - that only worked in the days before electric fans) and, if that does not seem to be working, shut it off.
Bohdan Bodnar wrote: "The 2.2/2.5 liter cooling systems *MUST* be purged of air before operation; otherwise, coolant flow blockage will result (i.e., hot, possibly REALLY hot, engine). Partial purging will cause the engine to run hotter than normal; the temperature will gradually drop to normal as the system purges itself over several days/weeks."
See this page for an easy guide to purging (bleeding) the radiator.
Here are some words on the issues involved and some fixes.
First, trapped pockets of air are a common problem, leading to a new procedure for changing antifreeze and bleeding the cooling system (click here for details).
Here's an interesting one: Louise Penberthy wrote that the clamp on the hose to the overflow bottle on her car wasn't tight enough; it had loosened during recent pressure-checking of the cooling system, letting air into the system.
Roger Fradenburgh "managed to trace a slow coolant leak to the point near the firewall where rubber coolant hoses are clamped to the metal ends of the heater core tubes. Tightening the clamps a few turns ended the problem. I probably never would've discovered the culprit had I not noticed that a nearby cable had an odd green-ish tint."
Tom Johnson wrote: "If the cooling system is low on water, the highest parts of the engine tend to overheat, causing the head to warp and the head gasket to blow out through the gaps left by the warping. Plymouth Reliants have temperature gauges and show a high reading within a few minutes of starting the engine IF it is low on coolant.
"If the heater/defroster fan doesn't blow toasty warm when the engine is hot, you are probably low on coolant. or have to bleed the system more. It helps to park the car on a grade (front end high), turn the heater temperature control up all the way, and idle the engine with the radiator cap off. Then, fill the cooling system. [Use distilled water - about fiftey cents a gallon from the supermarket]"
Mr. Schipp wrote: I have a '92 Grand Caravan that started overheating when my wife was driving from Harrisburg to Pittsburgh PA. It would only get real hot on the long uphill grades on the mountains and start puking coolant from the overflow tank. My brother-in-law replaced the thermostat when she got to Pittsburgh since it was the original with over 150,000 miles on it. The engine was still overheating on long uphill grades during the return trip. When she got back I checked the hoses first, they were fine. I checked the radiator for leaks, the heater hoses, the heater core, all were fine. But I still could not find the problem causing the overheating. ... The one part I overlooked, as I'm sure many home mechanics do, was the radiator cap! Since the cap was old, the spring was weak and when the pressure builds high enough it overcomes the spring pressure and allowed the coolant to flow from the radiator to the overflow tank. It just never got hot enough until under a good heavy load. Sometimes it's the obvious that escapes us!
From: "Frank E. Tressler Jr." <email@example.com> - Problem: Sundance thermometer swings back and forth. Repair attemps include replacing the thermostat and both engine temperature sensors. Test the fix by keeping the temp control on RECIRC and seeing if th gauge stays at a constant level. If so, check the hose going to from the thermostat housing to the heater core - it goes through a plastic bypass valve just before entering the heater core. The hoses on the valve turned out to have been switched at a prior servicing.
Robert Rowe: With the ignition on, ground the wire coming of the sending unit momentarily while the enging is warm. If the gauge moves to the correct temp, the sender is at fault. Do not ground for a long period of time as this can damage the gauge.
Peter Galambos: Temperature gauge would suddenly jump to 3/4 or almost full scale for a few seconds and then drop back to center. I hooked a voltmeter to the temperature sender input to the body computer and actually saw nice linear voltage swings. It appears that the gauge is designed to go super non-linear above a certain temperature. I disconnected the radiator fan long enough to verify that it was thermostat cycling causing the temperature swing and replaced the thermostat and antifreeze.
Dennis Lippert noted: The temperature gauge will swing back and forth until the entire system is warmed fully. This is because you keep introducing "cold" coolant from the radiator into the engine, rapidly cooling it. WHen the temp falls enough, the thermostat closes, and the temp goes back up, repeating the cycle. This is due to a valve which lets you get heat before the thermostat opens up. It keeps the warm coolant in the engine *and heater* when the thermostat was closed.
Peter Galambos related: Fixed by flushing the system with oxalic(sp?) acid (i.e. Prestone Super Flush). Now the heater works great and the engine temperature changes much less. There was probably a restriction even though the antifreeze looked fine; a lot of rust flowed out when flushing.
From: "Gene Yetter" <firstname.lastname@example.org> Problem: 1977 Volare Super Six water (heater) valve is leaking. The water valve is the small gold-colored item midway on the lower heater hose, and just next to the black valve cover breather. It has a vacuum lead. The leak is out of the bottom. The hose clamps actually remain dry. I'm tempted to try to apply some exterior sealant and wrap the valve with cloth, then tighten the wrap down with nylon ties. Do you think this might work for a few days assuming I keep after my coolant level? To replace the valve on the heater hose does the radiator have to be drained?
Scott, in response: I would just buy another heater hose and bypass the valve. All it does is allow the A/C to be cooler by blocking coolant flow through the heater core when you use the A/C. You can always get another valve later and then put it back the way it was. For now, your coolant leakage problem will be fixed. And no you don't have to drain the radiator - though you will lose some coolant when you do this - but you can always just refill the overflow bottle & radiator.
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